MAY 17, 2010: Longfellow Bridge over the Charles River . . . Free e-book for Globe subscribers: To download “Boston By Air” to your desktop, tablet, or mobile device (and learn how to buy prints of these photos), go to bostonglobe.com/insiders/ebook. You’ll find other free Globe e-books there as well. (Non-subscribers can purchase Boston By Air for $2.99 wherever e-books are sold.)